I first lined my pond with a layer of carpet foam.Then my first liner i bought was junk a 45mil pvc liner,I didnt like it so i ordered 45 mil epmd and place it right on top.The carpet padding prevented any large sharp rocks from poakin the liner.Only problem i can see is if your second liner gets a hole in it and drains to the first liner u put down it might lift up and bubble the second liner.